Vents only blow on HIGH on 2004 Highlander

Possible bad blend door actuator .some gm cars can be relearned to the new climate control head

Possibly the fresh air to re-circulation door is either binding or is broken.

your blend door motor is stuck or it may need to be replaced. the blend door controls the temp from heat to a/c

could be the blend door actuator.or the heat-A/c selector switch

the blend air door is stuck look down the heat vents to see if something fell in there

I'm no 'expert' but it sounds like possibly 2 different problems. Your heat comes from warm/hot coolant circulating through the heater core, and the fan blowing air through/over it. If your thermostat is not opening the coolant won't circulate. Thermostat is a not too difficult or costly starting place. No air flow leads me to think you have a blower motor issue, perhaps start with the fuse. Hope this helps.

Inside the ventilation system there is gates that open and close to direct the air flow. These are operated on vacuum on most vehicles. The hose for that perticular gate may have disconnected somehow or has a hole in it. I suggest you take the heater controls out and see if there is a plug with a series of colored hoses on it. it so you'll have to trace each of them to see which one is the problem. Now you may be able to get some help from you local dealers mechanic who just may know the color code for that gate. (which would be a bonus) Good Luck

You probably have a vacuum leak on the passenger side. its a small plastic hose some times they come unpluged from the engine. or brake.

Try an A/C recharge. What is the temps of the discharge air from the vents?

It may be the actuator not working properly. Should be above the accelerator pedal to the right.

Keeme